pgsql: doc: Improve pg_stat_recovery documentation

doc: Improve pg_stat_recovery documentation

Improve the documentation for pg_stat_recovery in several ways:

- Mention the view in high-availability.sgml as a way to monitor
recovery state and replay progress, alongside the existing recovery
information functions.
- Clarify that the view returns at most one row, not exactly one row,
and no rows to users who lack the pg_read_all_stats privilege.
- Correct the description of last_replayed_end_lsn to clarify that it
is the end LSN of the last replayed record plus one.
- Document that replay_end_tli equals last_replayed_tli when no WAL
record is currently being replayed.
- Clarify that current_chunk_start_time is NULL until streaming WAL
has been received.

Backpatch to v19, where pg_stat_recovery was introduced.

Author: Fujii Masao <masao(dot)fujii(at)gmail(dot)com>
Reviewed-by: Michael Paquier <michael(at)paquier(dot)xyz>
Discussion: https://postgr.es/m/CAHGQGwGRavm18HqnQn_f68QB96qk6arhjET1V93OJH09Mgojkg@mail.gmail.com
Backpatch-through: 19

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/937db82a8d6ffb1b3bb292ed7070dae33aa659ba

Modified Files
--------------
doc/src/sgml/high-availability.sgml | 13 +++++++++----
doc/src/sgml/monitoring.sgml | 33 +++++++++++++++++++--------------
src/include/access/xlogrecovery.h | 4 ++--
3 files changed, 30 insertions(+), 20 deletions(-)